Where are you located? Are you in the USA?

What country are you in? Regardless, you might has well tell him. It sounds like you will quit or be fired otherwise, so whatever chance if telling him why will sway him, or if you have legal protection, is no worse than that.

@bitmaker Welcome to The Workplace. I included an answer for you to consider. I admit I know near to nothing of Jewish traditions and customs, but, isn't there some sort of system or tool like the Eruv that you could consider so you can work this Saturday without compromising your beliefs? Again, I only know about the Eruv and that aids you with the traveling and carrying things part, don't know if there is an equivalent that aids for work. Mentioning this as a comment as I am not well equipped to include that in an answer... leaving it here FYI

@DarkCygnus Eruv only has to do with certain carrying by making the entire area covered as if it was one enclosed courtyard. (This is an extremely simplified explanation). Also, the OP might be Adventist or something else altogether. All off topic of course, and I know you were just trying to help so please take my comment in the same friendly spirit.

Does this answer your question? How to approach supervisor about religious accommodations?

@gnat completely disagree that the linked question is a Duplicate. Related? I also doubt it... that one is about Muslims and the praying they do throughout the day and this one is about Sabbath and communicating the manager about the real reasons why OP can't work that day... completely different things

Ask him to work Sunday.

Labour laws vary wildly from country to country. You need to specify the country and what your contract says. if your contract says you only work Monday-Friday, I'd stick to my contract.

As others have asked, we need the country (and if in the states, the specific state) you work in. Many jurisdictions have laws and regulations stating employers are required to give reasonable accommodations for deviation from the normal work agreement for religious reasons.

@Nzall OP is not asking for accommodations or deviations from the normal work schedule, the only one deviating is the boss.

Let's not mince words. You are certainly able to work on weekends. You just refuse to do so. Refusing to work is not a good look no matter how you spin it.

What are the possible disadvantages of mentioning it? I can't think of any. But if there are no disadvantages, then there's no question: obviously you should mention it.

@user91988 - a "good look" is far less important than respecting one's own work-life balance and religious convictions. It is reasonable to expect an employer to respect those boundaries, especially when they have been specified up front.

@user91988 "Refusing to work is not a good look..." This categorical statement is obviously wrong. I welcome you to think of counterexamples to it.

judaism.stackexchange.com/search?q=work+on+sabbath - I am assuming from the use of "sabbath" and "Saturday" that a link to Jewish guidance may be helpful to you. If I have guessed wrong, there are other religious sites on stackexchange which may help provide some specific advice, scripture, or counsel from a religious perspective on this issue.

@user76284 I disagree. What's your reasoning behind that? Saying something is "obviously wrong" doesn't mean anything. You have to provide reasons. See how that works?

@bitmaker you state you are not willing to "share more information about your religion" to your manager. I assume this also means your company's HR, if one exists. At that point, your only real options are work the Sabbath or be terminated for not working it without an explained (and good) reason. Also, I don't understand how you chose the answer as accepted that you did, as the only way that helps you is if you "share details with your manager" that you state you are unwilling to do.